Anion Trapping and Ionic Conductivity Enhancement in PEO-Based Composite Polymer-Li₇La₃Zr₂O₁₂ Electrolytes: The Role of the Garnet Li Molar Content

The successful development of all-solid-statebatterieswill providesolutions for many problems facing current Li-ion batteries, suchas high flammability, limited energy density, poor cyclability andlow cation transference number. In this quest, the development ofhigh-performance solid-state electrolytes is critical. Composite polymerelectrolytes (CPE), comprising ion-conducting (active) inorganic fillersand polymer matrices, have emerged as a promising strategy to yieldbetter conductivity, interfacial stability, and mechanical strengththan their single-phase counterparts. Recent experiments indicatethat active garnet fillers may enhance the ionic conductivity of CPEsby inducing anion trapping onto their surface. Moreover, substitutionsthat modify the lithium molar content within the filler were shownto impact this enhancement. However, the molecular underpinning behindthis phenomenon is poorly understood, hindering the development ofstrategies to exploit it optimally. In this study, we use an enhancedhybrid Monte Carlo technique in combination with extensive moleculardynamics simulations to bridge this gap. By focusing on the archetypalCPE formed by Ga-doped Li7-3x Ga x La3Zr2O12 (Ga- x -LLZO) embedded within a poly-(ethyleneoxide) (PEO) and lithium bis-(trifluoro-methane sulfonyl) imide(LiTFSI) polymer matrix, we describe how the dynamic electrostatictrapping of anions leads to overall conductivity enhancement by increasingthe lithium transference number and tracer diffusivity in the polymerphase. The extent of this enhancement can be fine-tuned by modulatingthe Li molar content of LLZO through the doping of Ga. We predictan optimal Li molar content of 5.95, which is lower than the optimal6.50 reported in the literature for single LLZO.